


Dipper vs Boobs

by AlexOC



Series: Cest is Best [1]
Category: Gravity Falls
Genre: Exploration, Extremely Underage, F/M, Sibling Incest
Language: English
Status: Completed
Published: 2016-04-01
Updated: 2016-04-01
Packaged: 2018-05-30 10:33:37
Rating: Explicit
Warnings: Underage
Chapters: 1
Words: 2,087
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/6420241
Author URL: https://archiveofourown.org/users/AlexOC/pseuds/AlexOC
Summary: <blockquote class="userstuff">
              <p>Apparently I write straight stuff now.<br/>After years of sharing a room with his sister, Dipper notices she's been changing.</p>
            </blockquote>





	Dipper vs Boobs

Dipper lay stretched out on top of his bed, Journal #4, a gift from uncle Ford last summer, open in front of him. The summer had been a hot one and Dipper hadn't even bothered with more than his shorts today, though somehow Mabel had kept up her endless wardrobe of sweaters.

Mabel sighed loudly, interrupting his latest journal entry. "I swear these things are going to make me throw out half my favorite shirts, nothing fits right anymore!" Dipper glanced over at her as she pulled of another sweater, exposing skin almost up to her armpits as it dragged her T-shirt almost off with it, and threw the thing into a pile with the other rejects. 

"What things?" Dipper said automatically, his eyes lingering on her as she straightened the pink T-shirt.

Without even blinking Mabel replied, “My boobs obviously.” Dipper’s jaw dropped open. “I mean I shouldn’t complain, but an extra inch or two can really mess with your wardrobe.”

“Mabel!” Dipper shouted glaring at her. “I don’t want to hear about your B-b-b,” he mumbled off not shout that at the top of his lungs. At least in front of his sister and with Stan downstairs.

“My Breasts,” Mabel said stretching out every syllable. “You don't’ have to be so weird about it, we share a room. Plus I listened when you started to grow hair on your-”

“Listened!” Dipper cut in blushing furiously. “You read my journal and then brought it up at breakfast!” He buried his face in the sheets. “Why is this happening…,” he moaned.

In a well practiced tone Mabel began, “It all begins with this little fella. The pituitary gl-” one of Dipper’s pillows bounced of her head.

“Oh come on Dip Dop these things aren’t exactly new,” Mabel said flopping back down onto her bed across the room from Dipper. “Heck I’ve even changed in front of you before, they can’t be that much of a shock.”

Dipper rolled over and stared up at the ceiling, still decorated with photos from their last summer. “It’s not like I was really on the lookout for them, you wear sweaters all year round, and you’re my sister not a...” He waves his hands frustratedly in the air.

“Flowering young woman?” Mabel suggested.

“Exactly” he said as they burst into laughter.

\-----------------------------------------------------------

Their laughter slowly died, and Dipper didn’t know what to say.

“I made it weird, didn’t I?” Mabel said after a minute.

“No!” he said quickly, taking a deep breath before continuing. “I mean, I know it’s all supposed to be normal, but it doesn’t feel right. Heck I don’t feel right and now you’re changing too.”

“We-” Mabel began hesitantly, “We could make it normal?”

“What?” Dipper said, looking over at her in confusion.

She looked back at him, blushing a little “Well you know? I’ve got questions, you’ve got question, I’ve got weird girl bits, you’ve got wierd boy bits. Maybe we could, um trade answers?”

He sat bolt upright, “Mabel! You don’t mean, I mean we can’t we’re...”

“Still sharing a room.” Mabel cut in, “It’s not like we shouldn’t have already seen it all anyways.”

Dipper had to admit it made a sort of sense, “I guess there is an extra room downstairs, and they never even asked if we wanted it. So they couldn’t really blame us if we accidently, just so happened to see each other... like that.”

“I doubt they planned it” Mabel said smiling, “but who are we to judge the mysterious plans of adults.” She let herself get a bit more serious “So what do we do? I could trade you my shirt for your shorts?”

“But I’d be naked!” Dipper blurted out, blushing again.

“Well that’s kind of the point.” she said with a smirk.

“Well then, if you’re keeping your skirt on, I’m putting a shirt on.” in a bit of a huff.

“I...” she blinked “ok, but it had better not be a long one. Deal?”

“Deal.”

Dipper hopped off his bed, rushed over to his suitcase and grabbed the first shirt he could find. He tugged the old red shirt over his head. It was a tight fit and barely came down to his waist.

“Together?” He asked as they both moved towards the center of the room.

“Yeah. One the count of three?” Mabel said shifting nervously.

“One...” They said together.

“Two...” He slid his thumbs into his briefs, she grabbed the bottom of her shirt a little.

\-----------------------------------------------------------

“Three!” Her brother said as he started to yank down his shorts.

Mabel was tempted to wait just a second or two; it wasn’t fair that she had to block her view to take of the shirt, and how often do you get to see your brother pants himself? But if she didn’t do it Dipper would be freaking out faster than you can say “hot 80s guys”.

She wiped the t-shirt over her head, and stared at her half naked brother.

It wasn’t exactly what she’d expected. It was bigger than last time she’d seen it, but it wasn’t anything like one’s from the magazines Grenda had leant her. It was soft, thin, and only a couple inches long. The hair surprised her too, not that it was there, but how little there was. Health class made it see like guys just woke up one day with a bush down there, Dipper wasn’t sparse but there was barely more than a square inch or two in the patch.

His dick twitched. 

She couldn’t help herself, “It’s kind of cute!” she said.

\-----------------------------------------------------------

“Three!” Mabel shouted, as she pulled her shirt up over her head.

Mabel stood straight backed in front of him, a smile on her face and trying to look confident. But the shirt she was twisting in both hands in front of her, something Dipper would love to be doing in the suddenly drafty room, gave her away.

Her skin was pale, with a tan line starting just below her neck. His eyes lingered on her unblemished skin before slowly trailing down for the full view.

They only stuck two inches maybe a little more, a gentle curve away from her body drawing to the almost point of her nipples.

It wasn’t the first time he’d “seen” a girl like this, his parents were careful about the internet, but he was better. He’d even fantasized about other girls he knew, Wendy and Pacifica had worn some pretty revealing stuff over his summers at Gravity Falls, but this was different.

This was more. Mabel was really here.

He couldn’t help himself, it just slipped from his lips, “Awesome...”

\-----------------------------------------------------------

“Cute?!” Dipper sputtered over, Mabel’s too proud “They are aren’t they!”

“You know that isn’t exactly what a guy wants to hear about his...”

“Penis?” Mabel said to Dipper’s awkward glare. "Oh come on, girls like cute and Little Dipper is definitely cute."

“Little?” Dipper sighed. “Mabel that’s even worse!”

“I’m only teasing, and if that’s how big it is in this cold drafty attic well...” Mabel trails off with a whistle.

“How do you even know about shrinkage?” Dipper pinched his nose “Nope, I really don’t want to know.”

"So when do I get to see Big Dipper?” Mabel said leaning towards him with a smirk.

"I'm not going to get... That! In front of you, Mabel!"

"It's perfectly natural Dipper, and I promise I’m not going to tell anyone."

“No!” Dipper almost shouted. “And you'd better not be planning on telling your friends about any of this!"

"I'll let you touch the twins?" 

His mouth hung open for a second.

“ok”

Mabel grabbed him by the wrists and pulled him towards his bed. She let herself fall backward onto the soft mattress nearly bringing Dipper crashing down on top of her in the process. He caught himself on hands and knees, almost pinning Mabel on the bed beneath him.

Dipper pushed himself up onto his knees, Mabel lay beneath him her bare chest puffing in and out. His pulse pounded in his ears, and he could feel himself growing hard growing hard as he inches his right hand hovered inches from her breasts.

Mabel looked up at him ‘Big Dipper’ in plain view, “Now that is awesome.”

She grabbed his outstretched hand pulling it closer, the tips of his fingers brushed against her and she let go.

He slid his fingers across her chest, before slowly dragging them down the center of her chest. Mabel had always been ticklish and she squirmed beneath him at the soft touch.

Her hands slid up his legs, and they bloom in goose bumps as she drew closer to his ass.

He didn’t know where this was going, but he couldn’t stop now. Dipper dove in with both hands, it took some experimentation but Mabel wasn’t hiding how he made her feel.

He had his hands all over her, applying gentle pressure feeling out her sensitivity. He didn’t know why but pushing them up from the bottom had her panting.

Her hands continued to explore him.

She must have been watching him before this. She knew exactly how to handle his dick; only a few seconds nearly had him over the edge. But it wasn’t long enough, she left him gasping as her hands slid beneath his shirt leaving him more and more exposed as they went.

Sweat beaded on their naked skin, their panting breaths and barely contained moans filled the room.

Dipper moved his hands to her nipples, they were hard between his fingers but that felt right. He started to rub them with his thumbs.

Mabel wrapped her arms around him, slowly pulling him closer and he felt his dick brush the inner folds of her skirt as she did.

His hands moved almost automatically now, his focus drawn up to the base of her neck. He leaned in closer, their eyes closed, his hot breath on her chest as he leaned in to kiss her.

“Dipper” Mabel moaned in pure please, the words almost rung in his ear.

The moment froze.

\-----------------------------------------------------------

They leapt off each other.

“What are we doing!” the words screamed in both their heads.

Dipper flailed at his shirt trying to pull it down, but it was bunched up and not long enough anyway. He desperately tried to cover up with his hands, but the sensitive state he was in didn’t make it easy or any less awkward. He back away as far as he could, but only made it a few steps before bumping into a wall.

Mabel pulled the covers up around her, “they smell like him...” She shook her head trying to think of something else. It was hard, Dipper was practically naked right in front of her and the room seemed so small all of a sudden.

“M-Mabel I-I- I’m going camping” Dipper stammered, one of his hands desperately feeling around him for his backpack.

“I NEED A SHOWER!” Mabel yelled as she bolted for the bathroom, wrapped in Dipper’s sheets.

\-----------------------------------------------------------

It was almost a week later. They hadn’t spoken since the incident and Dipper still didn’t know what to do. He’d been ‘camping’ the whole time, only visiting the shack for food and a shower. Mabel had spent the first two days at Grenda’s, but he’d seen her around the shack the last couple days.

Dipper stretched out in his sleeping bag he started at the last few pages of his book, he’d really need to grab another one next time he was in the shack, but his reading was interrupted by the rustle of the tent flap.

It was Mabel.

“Hi Bro.”

Dipper sat up to greet her, stumbling over the words, “m-Mabel hi, I didn’t really expect you…,” suddenly very thankful he’d decided to sleep in full PJs not his underwear.

She crawled into the tent sitting down right next to him.

“Grunkle Stan said to tell you that we’re eating early tonight, and we need to steer clear of the shack after 5. He’s got a date.”

“Oh, thanks Mabel.” He looked into her eyes before shying away. “I guess you’ll be hanging out with Grenda or something?”

She shrugged. “Well, I’ve got my sleeping bag right outside. I was thinking we could have a sleepover… If you want?”

He didn’t even have to think about it. “Of course I want to! Let’s just keep this one PG, ok.”

Mabel smiled back at him. “PG-13 and you’ve got a deal.”

Dipper held out his hand. “Deal.”

**Author's Note:**

> “Dipper vs Truth or Dare”  
> “Dipper vs The Other Girl”  
> “Mabel vs The Date”  
> “Dipper vs The Birthday Gift”
> 
> What do you think?


End file.
